Abstract. In order to study aerosol–cloud interactions in cirrus clouds, we apply a new
multiple-mode ice microphysical scheme to
the general circulation model ECHAM5-HAM.
The multiple-mode ice microphysical scheme allows for analysis of the competition
between homogeneous freezing of solution droplets, deposition nucleation of
pure dust particles, and immersion freezing of coated dust particles and
pre-existing ice. We base the freezing efficiencies of coated and pure dust
particles on the most recent laboratory data. The effect of pre-existing ice,
which has been neglected in previous ice nucleation parameterizations, is to
deplete water vapour by depositional growth and thus prevent homogeneous and
heterogeneous freezing from occurring. As a first step, we extensively tested
the model and validated the results against in situ measurements from various
aircraft campaigns. The results compare well with observations; properties
such as ice crystal size and number concentration as well as supersaturation are
predicted within the observational spread.
We find that heterogeneous
nucleation on mineral dust particles and the consideration of pre-existing
ice in the nucleation process may lead to significant effects: globally, ice
crystal number and mass are reduced by 10 and 5%, whereas the ice
crystals' size is increased by 3%. The reductions in ice crystal number are
most pronounced in the tropics and mid-latitudes in the Northern Hemisphere.
While changes in the microphysical and radiative properties of cirrus clouds
in the tropics are mostly driven by considering pre-existing ice, changes
in the northern hemispheric mid-latitudes mainly result from heterogeneous
nucleation. The so-called negative Twomey effect in cirrus clouds is
represented in ECHAM5-HAM. The net change in the radiation budget
is −0.94 W m−2, implying that both heterogeneous nucleation on dust
and pre-existing ice have the potential to modulate cirrus properties in
climate simulations and thus should be considered in future studies.